In a paper machine having a double-felted press for dewatering the web coming from a wet section of the machine, the wet paper web is passed through a press nip defined between two press members with the web sandwiched between a pair of absorbent press felts that also pass through the press nip. The web typically passes from the press nip to a further treatment unit, usually including one or more dryers, for further increasing the dry solids content of the web. In some types of paper machines, the web traverses an open draw between the press nip and the treatment unit. More particularly, the two press felts diverge from each other after the press nip, and the paper web separates from both of the felts and travels unsupported to the treatment unit, where it typically is picked up by a clothing of the treatment unit for carriage through the treatment unit.
One of the problems associated with such machines employing an open draw between a press nip and a treatment unit is that upon a web rupture occurring in the open draw, the leading end of the web created by the rupture tends to adhere to and travel with one of the press felts rather than separating from such felt as desired. This occurs because the forces on the web tending to separate it from the press felts are created by the web tension imparted by handling devices of the downstream treatment unit. When the web ruptures in the open draw, this web tension no longer acts on the web leading end, and thus the web tends to adhere to one of the felts.
Various prior art references disclose paper machines having double-felted press sections, but the known references do not address the above-noted problem associated with web rupture in an open draw. EP-0 598 991 describes a double-felted press section in a paper machine in which the upper press felt and lower press felt each travel about a guide roll, and wherein each guide roll is adjustable in relation to the paper web. During operation (see FIG. 3), the web is carried by the upper felt after the nip and transferred to the lower felt at a subsequent press. During operation, therefore, the web does not pass through any open draw.
EP-0 584 492 describes a system for threading a web in a press section that has a double-felted nip, but the system does not include an open draw.
U.S. Pat. No. 4,909,903 describes a double-felted shoe press, where the upper press felt and lower press felt each are guided about an adjustable guide roll and the web is free from contact with either of the felts after the press nip. The adjustable guide rolls are selectively positioned so as to vary an angle between the web and each of the two felts for controlling a licking-up process during which a leader of the web is threaded through the machine. The rolls are adjusted such that during the licking-up process, the wetter one of the felts is guided at a larger angle relative to the web, compared to the angle that exists during continuous operation, and thus the leader tends to separate from the wetter felt at a location closer to the press nip so as to minimize rewetting of the leader by the wetter felt. The patent does not disclose any devices or methods for controlling the path of travel of the web upon a rupture of the web in the free draw that follows the press nip.
GB-1 284 119 describes a double-felted press nip, where the upper press felt has a guide roll in its loop which is moved to a lower position to deflect the upper felt into contact with the lower felt during threading. During operation, the web is carried by the lower felt. There is no open draw in this known configuration.
DE-195 11 988 shows a press section that has a double-felted shoe press (FIG. 2), where the upper press felt runs around an adjustable guide roll located downstream of the press nip. A pick-up roll is arranged a relatively short distance from the counter roll of the shoe press. The adjustable guide roll can be set in a plurality of different positions, in some of which the upper press felt carries the web up to the pick-up roll. In one position of the adjustable guide roll, the upper press felt is caused to stand clear of the web so that the web runs in an open draw. The problem of web rupture in the open draw is not addressed in the patent specification, nor is any method mentioned or device shown to deal with this problem.